1. Field of the Invention
The present invention relates to an apparatus for applying thermoplastic hot melt glue, and more particularly to such an apparatus which is capable of preventing hot melt from roping from a nozzle when the nozzle is shut off. The present invention further relates to a nozzle through which viscous fluid such as glue, paint and sealing material is discharges and which is suitable for the above mentioned apparatus.
2. Description of the Prior Art
Boxes and bags have been conventionally made by intermittently applying a glue onto a glue margin of boxes and bags, folding the glued margins to thereby secure the margins to a predetermined areas, and thus completing boxes and bags. Glue is applied by means of a roller or a discharge nozzle. When glue is to be intermittently applied, a discharge nozzle has been widely used because of easiness of control thereof.
In the case that a discharge nozzle is to be used to intermittently discharge glue, when glue discharge is shut off, glue discharge does not stop immediately, and hence glue tends to make glue filament. In order to solve this problem, Japanese Unexamined Patent Publication No. 61-78460 has suggested the production of negative pressure in a nozzle when a applicator nozzle is shut off, to thereby prevent making of glue filament. Japanese Unexamined Patent Publications Nos. 50-122539 and 55-2474 have suggested the provision of an absorbing nozzle in a discharge nozzle to thereby absorb glue which remains non-discharged when the discharge nozzle is shut off. Japanese Unexamined Patent Publication No. 5-97127 has suggested that when a nozzle opening is shut off, a glue discharge speed is increased and glue is blown off by discharging air through an air nozzle to thereby prevent making of glue filament.
Glue is grouped into water base glue which has been generally used and is called cold glue, and thermoplastic glue which is called hot melt. With respect to general characteristics, cold glue takes time for drying until an adhesive force is generated after cold glue has been applied, whereas hot melt produces an adhesive force immediately after having been applied. Hot melt has great viscosity, and hence, when hot melt is to be applied through a nozzle, hot melt tends to make glue filament at the time a nozzle opening is shut off. The applicator disclosed in Japanese Unexamined Patent Publication No. 5-97127 is effective for cold glue to prevent making of glue filament, but could not prevent making filament of hot melt.
As mentioned earlier, hot melt produces an adhesive force immediately after having been applied. Since hot melt is in a solid condition at room temperature, hot melt is heated to thereby be liquidized for applying through a discharge nozzle. However, a part of the liquid is gasified by heating, and the thus produced gas tends to adhere to a piston of a pneumatic cylinder for driving a discharge nozzle for open and close action thereof and, after cooled, fixedly secure a piston to a cylinder.
There has been used a discharge nozzle for releasing the above mentioned hot melt gas to atmosphere to thereby prevent the gas from entering a piston. FIG. 1 illustrates an example of a discharge nozzle formed with an opening through which hot melt gas is to be released to atmosphere. A main body 1 of the illustrated discharge nozzle is partitioned into a glue chamber 3 and a cylinder 4 by a sealing section 2. There are provided a glue chamber cover 5 having a nozzle opening 9, and a glue inlet 11 in the glue chamber 3. Glue supplied through the glue inlet 11 is discharged through the nozzle opening 9. A piston 7 is slidably fit in the cylinder 4. A valve rod 8 extends passing through the sealing section 2, and has both a tip end 8a for opening and closing the nozzle opening 9 and a rear end at which the valve rod is connected to the piston 7. There is provided a spring 10 at the rear of the piston 7, namely at the opposite side of the valve rod 8, which spring compresses the piston 7 so that the nozzle opening 9 is closed with the tip end 8a of the valve rod 8. Within the cylinder 4 is provided an air supply inlet 16 at the side of the sealing section 2. Air under pressure is supplied into the glue chamber 3 through the air supply inlet 16 to move the piston to thereby separate the tip end 8a of the valve rod 8 from the nozzle opening 9 for discharging glue therethrough.
The sealing section 2 is formed centrally with a valve rod passage 12 through which the valve rod 8 passes, and with an atmosphere releasing passage 17 which communicates the valve rod passage 12 to atmosphere. At opposite ends of the valve rod passage 12 are provided U-shaped seals 14 for preventing glue from entering the cylinder 4 from the glue chamber 3. When the valve rod 8 makes reciprocal movement, fluid glue acting as a lubricant enters the valve rod passage 12 in a small amount and is gasified. A part of the thus produced gas is released through the atmosphere releasing passage 17.
As discussed earlier, even if a structure including two seals and an atmosphere releasing passage located intermediate between the two seals is used, gasified glue enters a cylinder during a discharge nozzle is used for long hours, and adheres to a slide surface of a piston, resulting in that a piston is fixedly adhered to a cylinder. In particular, when a discharge nozzle is to be used in horizontally lying condition or with a nozzle being upwardly directed, a piston frequently is fixedly adhered to a cylinder in a relatively short time.